The overall objective of this program is to exploit mutagen-sensitive strains of Drosphila melanogaster to obtain a better understanding of the mechanism of eukaryotic DNA metabolism. The following work is proposed. (A) Using larval brain ganglia in four mutants, by-pass synthesis will be analyzed. This analysis is made possible by a mating scheme which employs a dominant temperature sensitive mutant to select against heterozygotes. This scheme will also allow sterile alleles of previously identified mutants to be analyzed. (B) The kinetics of chain elongation will be determined in previously identified strong by-pass defective mutants. (C) The effect of UV on chain growth, and replicon size in normal, excision-defective, and in by-pass defective mutants will be analyzed by fiber autoradiography. (D) The efficient photoreactivation system of Drosophila will be exploited to measure the effect of photoreactivating light on the UV-induced depression in thymidine incorporation and molecular weight of nascent DNA. (E) Using larval brain ganglia, stocks, which must be maintained in heterozygous form will be tested for hypersensitivity to aphidicolin, an inhibitor of alpha-polymerase. (F) By using thin-layer chromatography, both pool sizes of deoxyribonucleotides and ribonucleotide reduction will be measured to determine if DNA synthesis is affected by altered levels of deoxyribonucleotides in any available mutants. (G) The alpha-polymerase assay will be used to characterize mutants showing reduced synthesis and hypertensitivity to aphidicolin. (H) Analysis of unscheduled DNA synthesis will be extended to additional mutants. (I) Using autoradiographic techniques, we will determine quantitatively whether injected diploid nuclei have been induced to initiate DNA synthesis by the host salivary nucleus.